If you're burning a full disc, and it slows down near the end, it probably is a media problem. The drive has to slow down to guarantee the best write.
Try using Nero CD Speed and "create data disc" from the test menu. It should run a smooth linear curve to the end of the disc. If it drops off at/near the end, the media is poor quality
If you're burning a couple of tracks, and it slows down on the last one, it's something in your system.
Also, the drive slows down to write lead-in and TOC at the end of the write if you're doing track at once.